The move was widely expected after Shinsei's board this month opted to use a poison pill by issuing warrants to existing shareholders that would dilute SBI's stake.

SBI,which owns Japan's largest online brokerage, operates an online bank and has stakes in regional banks, has said it planned to lift its stake in Shinsei to as much as 48% from 20%.

(Reporting by David Dolan, Editing by Louise Heavens)